In an alternating current (AC) electrical system that is not purely resistive (i.e., having a capacitive and/or inductive component), power can be stored in the load and then unproductively returned to the grid. The associated current results in an undesirable loss in energy in the system. The extent of this loss is represented by the power factor of the system, defined as true power/apparent power and is a dimensionless ratio indicative of how efficiently current is being converted to real power.
Standards created by the International Electrotechnical Commission such as the IEC-1000-3-2 Int. Std., 2001, and later adopted as regional standards as EN-61000-3-2, were created to regulate the amount of permissible harmonic content generated by grid-connected electrical devices. By actively controlling the AC line current to be sinusoidal and in-phase with the AC line voltage, commonly known as power factor correction (PFC), the total harmonic distortion (THD) of current can be reduced and the power factors of these electronic devices can be increased thereby meeting these recommendations. Complementing the rise in popularity of these standards, advances in digital control techniques and digital devices has enabled performance and cost advantages over analog controllers and techniques (for instance D. Maksimovic et al., “Impact of digital control in power electronics,” in Proceedings 16th International Symposium on Power Semiconductor Devices and ICs, 2004, pp. 13-22). The importance of meeting such international energy standards and programs, to meet efficiency, input current harmonic and/or power factor requirements, has necessitated the development of advanced circuits and control techniques allowing compliance with these increasingly aggressive limits. For consumer devices and electronics operating at low powers, a boost PFC converter, shown in FIG. 1, is a popular active topology used for PFC and pre-regulation (R. W. Erickson et al., Fundamentals of Power Electronics, Kluwer Academic Publishers, Secaucus, N.J., USA, 2001). Its dynamics are governed by the behavior of the inductor current during a single switching cycle, and the three primary modes of operation are discontinuous conduction mode (DCM), boundary conduction mode (BCM), and continuous conduction mode (CCM). Over multiple switching cycles, if the inductor current operates in a combination of CCM, BCM, or DCM, there is a fourth mode known as mixed-conduction mode (MCM) (J. Sebastian et al., “The determination of the boundaries between continuous and discontinuous conduction modes in PWM DC-to-DC converters used as power factor pre-regulators,” IEEE Trans. Power Electron., vol. 10, no. 5, pp. 574-582, September 1995., and K. De Gusseme et al., “Sample correction for digitally controlled boost PFC converters operating in both CCM and DCM,” in Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE, 2003, pp. 389-395 vol. 1). If the inductor current remains above zero for the full duration of the switching period, the converter is operating in continuous conduction mode (CCM). If, however, the inductor current falls to zero and remains at zero for a portion of the switching period, the mode of operation is known as discontinuous conduction mode (DCM). Generally, DCM is reserved for power levels under 300 W, and hence lower peak currents, due to its high current ripple, and sometimes variable switching frequency, necessitates more complex electromagnetic interference filtering (L. Huber et al., “Performance evaluation of bridgeless PFC boost rectifiers,” IEEE Trans. Power Electron., vol. 23, no. 3, pp. 1381-1390, May 2008). For higher power levels, CCM boost PFC converters may still operate in DCM at high-line and/or light loads. If the boost PFC converter operates in both CCM and DCM during a half-line cycle, it is said to be operating in mixed-conduction mode (MCM) (D. M. Van de Sype et al., “Duty-ratio feedforward for digitally controlled boost PFC converters,” IEEE Trans. Ind. Electron., vol. 52, no. 1, pp. 108-115, February 2005). Each conduction mode requires different control considerations, and it is therefore desirable to detect the mode of operation, and/or the moment of zero inductor current for proper control.
Existing methods to detect zero inductor current employ auxiliary windings to monitor the voltage across the boost inductor (Fairchild Semiconductor, “FAN7930 Critical Conduction Mode PFC Controller,” FAN7930, April 2010), or use ancillary methods, either digital techniques or analog circuits, to detect DCM or zero current detection (ZCD). With auxiliary windings, there is added bulk and cost to install an auxiliary winding to the boost inductor. Furthermore, if MCM control techniques wish to be explored without significant hardware modification, replacing existing inductors with multi-winding inductors is impractical if the product design is complete. Detection of the DCM boundary using numerical computation is also possible, but with increased sensitivity to passive component tolerances. A digital DCM detection method is presented in US patent application 2011/211377, which decides the mode of operation based on a comparison of inductor current samples, requiring two current samples with an analog-to-digital converter (ADC) in a single switching period. Other digital detection approaches, such as the ones proposed in T. Hwang et al., “Seamless boost converter control in critical boundary condition for fuel cell power conditioning system,” Energy Conversion Congress and Exposition (ECCE), 2011 IEEE, September 2011, pp. 3641-3648 and T. Hwang et al., “Seamless Boost Converter Control Under the Critical Boundary Condition for a Fuel Cell Power Conditioning System,” IEEE Trans. Power Electron., vol. 27, no. 8, pp. 3616-3626, 2012, require prior knowledge of the boost inductance, immediate output and input voltages, as well as the inductor current. Accurate DCM detection is provided for instance in S. Moon et al., “Accurate mode boundary detection in digitally controlled boost power factor correction rectifiers,” Energy Conversion Congress and Exposition, 2010 IEEE, 2010, pp. 1212-1217 and S. Moon et al., “Autotuning of Digitally Controlled Boost Power Factor Correction Rectifiers,” IEEE Trans. Power Electron., vol. 26, no. 10, pp. 3006-3018, 2011, through use of an auxiliary injection circuit and digital computation. All of these existing methods, however, suffer from either a need of, or a combination of, auxiliary circuits, fast ADCs, a dependency on component values, or increased computational requirements.
An important consideration of the boost PFC converter depicted in FIG. 1 is the behavior of its inductor current during a single switching cycle. The CCM and DCM boost PFC converters have significantly different small-signal dynamics. Therefore, if the boost PFC converter is designed for CCM operation, but instead operates in DCM, it will show increased input current distortion, possibly compromising its harmonic limits. Consequently, much interest in the control of MCM boost PFC converter has been seen recently, with significant emphasis on digital control techniques to overcome the challenges of traditional analog control techniques. Digital feedforward control was proposed for MCM control in for instance K. De Gusseme et al., “Digitally controlled boost power-factor-correction converters operating in both continuous and discontinuous conduction mode,” IEEE Trans. Ind. Electron., vol. 52, pp. 88-97, 2005, and then later adapted for predictive control in L. Roggia et al., “Digital control system applied to a PFC boost converter operating in mixed conduction mode,” in Power Electronics Conference COBEP '09, Brazilian, pp. 698-704. Feedforward control in K. De Gusseme et al., “Digitally controlled boost power-factor-correction converters operating in both continuous and discontinuous conduction mode,” IEEE Trans. Ind. Electron., vol. 52, pp. 88-97, 2005 and L. Roggia et al., “Digital control system applied to a PFC boost converter operating in mixed conduction mode,” in Power Electronics Conference, COBEP '09, Brazilian, pp. 698-704 requires two separate feedforward actions, either the DCM or CCM duty cycle, to determine the appropriate control law. Upon computation and selection of the minimum feedforward term, a suitable duty cycle is applied, providing a decrease in THD and increase power factor over the non-feedforward controller. A drawback of this feedforward control technique is the need for comparison and computation of two separate feedforward terms, although ultimately, only a single term is used for the controller output. Furthermore, the DCM duty cycle feedforward term requires both division and square root operations, demanding an increased number of digital instruction cycles. The authors in F. Chen et al., “Digital Control for Improved Efficiency and Reduced Harmonic Distortion Over Wide Load Range in Boost PFC Rectifiers,” IEEE Trans. Power Electron., vol. 25, pp. 2683-2692, 2010 approach the control of the MCM boost converter by using an auxiliary winding on the boost inductor and a voltage comparator to measure the length of the DCM period. Adaptive and predictive control techniques are used to realize THD and power factor over average current mode control with positive results. Again, a disadvantage however is the need for an auxiliary inductor winding to detect zero inductor current and DCM operation. In C. Liou et al., “Design and implementation of a boost power factor correction stage operated in mixed-conduction mode,” in International Conference on Electric Information and Control Engineering, 2011, pp. 2069-2072, the authors propose sensing the load current and deciding CCM or DCM operation based on a digital computation, thereby allowing MCM control. Sensing the load current, however, results in efficiency penalties as well as the need for two external comparators and a logical AND gate. Further, in the aforementioned T. Hwang et al., “Seamless Boost Converter Control Under the Critical Boundary Condition for a Fuel Cell Power Conditioning System, a DSP is used for numerical detection of DCM operation, which is based on known component values and sensing on the input and output voltages, as well as the inductor and output currents. MCM control is provided, but is subjected to increased computational and cost requirements due to the sampling of four quantities, as well as needing known values for computation of the DCM condition.
Thus, there remains a need for simplified and improved devices and methods for detecting a DCM condition in a power factor correction circuit and for MCM operation.